Split library

[accosmin]

• libtensor (basically the tensor namespace)
• liboptim (basically the optim namespace)
• libutil (non machine learning-related functionality)
• libmachine (the machine learning part)

Also think of better names for these modules. Each should have its own CMakeLists.txt and an install target.

[accosmin]

Another suggestion: cmin, ctensor, core, cnet

[accosmin]

Also use different namespaces for each library.